The present disclosure pertains to a process for preparing aqueous encapsulated pigment dispersions. These dispersions may be used in the preparation of aqueous inkjet inks, pigmented paints, and the like. More particularly, the disclosure relates to a dispersion process where a dispersed pigment is prepared with the amount of polymeric dispersant that is not bound to the pigment minimized, then monomers are added to the dispersed pigment, and polymerization is initiated to obtain an encapsulated pigment which has minimal free polymer in the final encapsulated pigment dispersion.
Dispersion methods are commonly used to disperse particles. A variety of dispersion processes have been described for dispersing particles. For inkjet inks pigments are dispersed with polymeric dispersants. Recently, processes have been described where the polymeric dispersed pigments are subject to a process that results in encapsulated pigments.
In U.S. Pat. No. 6,262,152 describes preparing encapsulated pigments where the encapsulation is achieved by using polymeric dispersants which have reactive sites and adding monomers that have substituents that can react with the reactive sites of the polymeric dispersant. For instance, the dispersant has isocyanate reactive groups and an isocyanate is added to react with the polymeric dispersant.
U.S. Pat. No. 7,612,124 describes polymer-enclosed color-imparting particles that are enclosed by a friable polymer formed from a water-dispersible polymerizable material.
U.S. Pat. No. 7,741,384 describes an encapsulation process where both the pigment dispersion and the monomer dispersion are separately mixed utilizing a surfactant to independently stabilize the dispersion and the monomer.
US Patent Application Publication No. 200700227401 describes an encapsulation process where the monomer is stabilized via a miniemulsion which is stabilized by a hydrophobic organic compound.
US Patent Application Publication No. 20080064786 describes a water insoluble polymeric dispersant for the pigment and two crosslinking steps, the first occurring in a ketone/water solvent and crosslinks the core of the polymer followed by a second crosslinking for the shell of the polymer.
There have been efforts in the art directed at improving the stability of pigment dispersions. These efforts have included improvements in the processes used to make the dispersions, the development of new dispersants and the exploration of the interaction between dispersants and pigment particles, and between dispersants and the aqueous vehicle. While much of the effort has general application at improving dispersion stability, some of that effort has not found utility in particular applications. For example, the pigment dispersions used in inkjet printing applications have very unique and demanding requirements. It is critical that ink components comprising pigment dispersion remain stable, not only in storage but also over repeated jetting cycles. It is also desirable that the pigment dispersions offer good durability, good rub-fastness, wet-fastness and highlighter pen fastness. As the inkjet industry moves to page-wide array printing the requirements for repeating jetting cycles may be an order of magnitude higher than the traditional Small Office/Home Office market. These and other emerging needs require improved pigment dispersions.
A need exists for highly stable, higher-quality and different property inks for ink-jet applications. Although improvements in polymeric dispersants have significantly contributed to improved ink-jet inks, the current dispersants still do not provide inks with the requisite stability, durability, optical density and chroma needed for inkjet applications. The present invention satisfies this need by providing an encapsulated pigment dispersion based on a pigment which is stabilized by a polymeric dispersant to which is added acrylate monomers and optionally acrylic or vinyl monomers, which are, in turn, polymerized. This results in an encapsulated pigment dispersion that has more stability, better jetting without compromise of color properties and more flexibility for ink formulation.